 @@ -1,1686 +1,1686 @@
-.\"	$NetBSD: options.4,v 1.369.2.2 2008/11/18 01:56:58 snj Exp $
+.\"	$NetBSD: options.4,v 1.369.2.2.6.1 2013/06/19 07:59:02 bouyer Exp $
 .\"
 .\" Copyright (c) 1996
 .\" 	Perry E. Metzger.  All rights reserved.
 .\"
 .\" Redistribution and use in source and binary forms, with or without
 .\" modification, are permitted provided that the following conditions
 .\" are met:
 .\" 1. Redistributions of source code must retain the above copyright
 .\"    notice, this list of conditions and the following disclaimer.
 .\" 2. Redistributions in binary form must reproduce the above copyright
 .\"    notice, this list of conditions and the following disclaimer in the
 .\"    documentation and/or other materials provided with the distribution.
 .\" 3. All advertising materials mentioning features or use of this software
 .\"    must display the following acknowledgment:
 .\"	This product includes software developed for the NetBSD Project
 .\"	by Perry E. Metzger.
 .\" 4. The name of the author may not be used to endorse or promote products
 .\"    derived from this software without specific prior written permission.
 .\"
 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
 .\"
-.Dd November 14, 2008
+.Dd May 15, 2013
 .Os
 .Dt OPTIONS 4
 .Sh NAME
 .Nm options
 .Nd Miscellaneous kernel configuration options
 .Sh SYNOPSIS
 .Cd cinclude ...
 .Cd config ...
 .Cd [no] file-system ...
 .Cd ident ...
 .Cd include ...
 .Cd [no] makeoptions ...
 .Cd maxusers ...
 .Cd [no] options ...
 .Cd [no] pseudo-device ...
 .Sh DESCRIPTION
 This manual page describes a number of miscellaneous kernel
 configuration options that may be specified in a kernel config
 file.
 See
 .Xr config 1
 and
 .Xr config 5
 for information on how to configure and build kernels.
 .Pp
 The
 .Ar no
 form removes a previously specified option.
 .Ss Keywords
 The following keywords are recognized in a kernel configuration file:
 .Bl -ohang
 .It Sy cinclude Xo
 .Qq Ar filename
 .Xc
 Conditionally includes another kernel configuration file whose name is
 .Ar filename ,
 which may be double-quoted and may be an explicit path or relative to
 the kernel source directory.
 Failure to open the named file is ignored.
 .It Sy config Xo
 .Ar exec_name
 .No root on
 .Ar rootdev
 .Op type Ar fstype
 .Op dumps on Ar dumpdev
 .Xc
 Defines a configuration whose kernel executable is named
 .Ar exec_name ,
 normally
 .Dq netbsd ,
 with its root file system of type
 .Ar fstype
 on the device
 .Ar rootdev ,
 and optionally specifying the location of kernel core dumps on the device
 .Ar dumpdev .
 .Ar dev
 or
 .Ar dumpdev
 and
 .Ar fstype
 may be specified as
 .Dq \&? ,
 which is a wild card.
 The root
 .Ar fstype
 and
 .Ar dumpdev
 are optional and assumed to be wild carded if they are not specified.
 .It Ar device_instance Xo
 at
 .Ar attachment
 .Op Ar locators value Op ...
 .Op flags Ar value
 .Xc
 Define an instance of the device driver
 .Ar device_instance
 that attaches to the bus or device named
 .Ar attachment .
 An
 .Ar attachment
 may require additional information on where the device can be found, such
 as an address, channel, function, offset, and/or slot, referred to as
 .Ar locators ,
 whose
 .Ar value
 often may be a wild card,
 .Dq \&? .
 Some device drivers have one or more
 .Ar flags
 that can be adjusted to affect the way they operate.
 .It Sy file-system Xo
 .Ar fs_name Op , Ar fs_name Op ...
 .Xc
 Include support for the file-system
 .Ar fs_name .
 .It Sy ident Xo
 .Qq Ar string
 .Xc
 Sets the kernel identification string to
 .Ar string .
 .It Sy include Xo
 .Qq Ar filename
 .Xc
 Functions the same as
 .Ar cinclude ,
 except failure to open
 .Ar filename
 produces a fatal error.
 .It Sy options Xo
 .Ar option_name Op , Ar option_name=value Op ...
 .Xc
 Specifies (or sets) the option, or comma-separated list of options,
 .Ar option_name .
 Some options expect to be assigned a value, which may be an integer,
 a double-quoted word, a bare word, or an empty string
 .Pq Qq .
 Note that those are eventually handled by the C compiler, so the rules
 of that language apply.
 .Pp
 .Em Note :
 Options that are not defined by device definition files are passed to
 the compile process as
 .Fl D
 flags to the C compiler.
 .It Sy makeoptions Xo
 .Ar name=value
 .Xc
 Defines a
 .Xr make 1
 macro
 .Ar name
 with the value
 .Ar value
 in the kernel Makefile.
 .It Sy maxusers Xo
 .Ar integer
 .Xc
 Set the maxusers variable in the kernel.
 .It Sy no Xo
 .Ar keyword Ar name Op Ar arguments Op ...
 .Xc
 For the
 .Xr config 1
 .Ar keywords
 file-system, makeoptions, options, and pseudo-device,
 .Ar no
 removes the file-system, makeoption, options, or pseudo-device,
 .Ar name .
 This is useful when a kernel configuration file includes another which
 has undesired options.
 .Pp
 For example, a local configuration file that wanted the kitchen sink, but
 not COMPAT_09 or bridging, might be:
 .Bd -literal -offset indent
 include "arch/i386/conf/GENERIC"
 no options COMPAT_09
 no pseudo-device bridge
 .Ed
 .It Sy pseudo-device Xo
 .Ar name
 .Op Ar N
 .Xc
 Includes support for the pseudo-device
 .Ar name .
 Some pseudo-devices can have multiple or
 .Ar N
 instances.
 .El
 .Ss Compatibility Options
 .Bl -ohang
 .It Cd options COMPAT_09
 Enable binary compatibility with
 .Nx 0.9 .
 This enables support for
 -bit user, group, and process IDs (following revisions support
 -bit identifiers).
 It also allows the use of the deprecated
 .Xr getdomainname 3 ,
 .Xr setdomainname 3 ,
 and
 .Xr uname 3
 syscalls.
 This option also allows using numeric file system identifiers rather
 than strings.
 Post
 .Nx 0.9
 versions use string identifiers.
 .It Cd options COMPAT_10
 Enable binary compatibility with
 .Nx 1.0 .
 This option allows the use of the file system name of
 .Dq ufs
 as an alias for
 .Dq ffs .
 The name
 .Dq ffs
 should be used post 1.0 in
 .Pa /etc/fstab
 and other files.
 It also adds old syscalls for the
 .At V
 shared memory interface.
 This was changed post 1.0 to work on 64-bit architectures.
 This option also enables
 .Dq sgtty
 compatibility, without which programs using the old interface produce
 an
 .Dq inappropriate ioctl
 error, and
 .Pa /dev/io
 only works when this option is set in the kernel,
 see
 .Xr io 4
 on ports that support it.
 .It Cd options COMPAT_11
 Enable binary compatibility with
 .Nx 1.1 .
 This allows binaries running on the i386 port to gain direct access to
 the io ports by opening
 .Pa /dev/io
 read/write.
 This functionality was replaced by
 .Xr i386_iopl 2
 post 1.1.
 On the
 .Tn Atari
 port, the location of the disk label was moved after 1.1.
 When the
 .Em COMPAT_11
 option is set, the kernel will read (pre) 1.1 style disk labels as a
 last resort.
 When a disk label is re-written, the old style label will be replaced
 with a post 1.1 style label.
 This also enables the
 .Em EXEC_ELF_NOTELESS
 option.
 .It Cd options COMPAT_12
 Enable binary compatibility with
 .Nx 1.2 .
 This allows the use of old syscalls for
 .Fn reboot
 and
 .Fn swapon .
 The syscall numbers were changed post 1.2 to add functionality to the
 .Xr reboot 2
 syscall, and the new
 .Xr swapctl 2
 interface was introduced.
 This also enables the
 .Em EXEC_ELF_NOTELESS
 option.
 .It Cd options COMPAT_13
 Enable binary compatibility with
 .Nx 1.3 .
 This allows the use of old syscalls for
 .Fn sigaltstack ,
 and also enables the old
 .Xr swapctl 2
 command
 .Dv SWAP_STATS
 (now called
 .Dv SWAP_OSTATS ) ,
 which does not include the
 .Fa se_path
 member of
 .Va struct swapent .
 .It Cd options COMPAT_14
 Enable binary compatibility with
 .Nx 1.4 .
 This allows some old
 .Xr ioctl 2
 on
 .Xr wscons 4
 to be performed, and allows the
 .Dv NFSSVC_BIOD
 mode of the
 .Xr nfssvc 2
 system call to be used for compatibility with the deprecated nfsiod program.
 .It Cd options COMPAT_15
 Enable binary compatibility with
 .Nx 1.5 .
 Since there were no API changes from
 .Nx 1.5
 and
 .Nx 1.6 ,
 this option does nothing.
 .It Cd options COMPAT_16
 Enable binary compatibility with
 .Nx 1.6 .
 This allows the use of old signal trampoline code which has been deprecated
 with the addition of
 .Xr siginfo 2 .
 .It Cd options COMPAT_20
 Enable binary compatibility with
 .Nx 2.0 .
 This allows the use of old syscalls for
 .Fn statfs ,
 .Fn fstatfs ,
 .Fn getfsstat
 and
 .Fn fhstatfs ,
 which have been deprecated with the addition of the
 .Xr statvfs 2 ,
 .Xr fstatvfs 2 ,
 .Xr getvfsstat 2
 and
 .Xr fhstatvfs 2
 system calls.
 .It Cd options COMPAT_30
 Enable binary compatibility with
 .Nx 3.0 .
 See
 .Xr compat_30 8
 for details about the changes made after the
 .Nx 3.0
 release.
 .It Cd options COMPAT_43
 Enables compatibility with
 .Bx 4.3 .
 This adds an old syscall for
 .Xr lseek 2 .
 It also adds the ioctls for
 .Dv TIOCGETP
 and
 .Dv TIOCSETP .
 The return values for
 .Xr getpid 2 ,
 .Xr getgid 2 ,
 and
 .Xr getuid 2
 syscalls are modified as well, to return the parent's PID and
 UID as well as the current process's.
 It also enables the deprecated
 .Dv NTTYDISC
 terminal line discipline.
 It also provides backwards compatibility with
 .Dq old
 SIOC[GS]IF{ADDR,DSTADDR,BRDADDR,NETMASK} interface ioctls, including
 binary compatibility with code written before the introduction of the
 sa_len field in sockaddrs.
 It also enables
 support for some older pre
 .Bx 4.4
 socket calls.
 .It Cd options COMPAT_BSDPTY
 This option is currently on by default and enables the pty multiplexer
 .Xr ptm 4
 and
 .Xr ptmx 4
 to find and use ptys named
 .Pa /dev/ptyXX
 (master) and
 .Pa /dev/ttyXX
 (slave).
 Eventually this option will become optional as ptyfs based pseudo-ttys become
 the default, see
 .Xr mount_ptyfs 8 .
 .It Cd options COMPAT_SVR4
 On those architectures that support it, this enables binary
 compatibility with
 .At V.4
 applications built for the same architecture.
 This currently includes the i386, m68k, and sparc ports.
 .It Cd options COMPAT_LINUX
 On those architectures that support it, this enables binary
 compatibility with Linux ELF and
 .Xr a.out 5
 applications built for the same architecture.
 This currently includes the alpha, arm, i386, m68k, mips, powerpc and
 x86_64 ports.
 .It Cd options COMPAT_LINUX32
 On those 64 bit architectures that support it, this enables binary
 compatibility with 32 bit Linux binaries.
 For now this is limited to running i386 ELF Linux binaries on amd64.
 .It Cd options COMPAT_SUNOS
 On those architectures that support it, this enables binary
 compatibility with
 .Tn SunOS 4.1
 applications built for the same architecture.
 This currently includes the sparc, sparc64 and most or all m68k ports.
 Note that the sparc64 requires the
 .Em COMPAT_NETBSD32
 option for 64-bit kernels, in addition to this option.
 .It Cd options COMPAT_ULTRIX
 On those architectures that support it, this enables binary
 compatibility with
 .Tn ULTRIX
 applications built for the same architecture.
 This currently is limited to the pmax.
 The functionality of this option is unknown.
 .It Cd options COMPAT_DARWIN
 On those architectures that support it, this enables binary compatibility with
 .Tn Darwin
 applications built for the same architecture.
 This feature is highly experimental, it requires COMPAT_MACH and
 EXEC_MACHO and it is currently limited to i386 and powerpc ports of
 .Nx .
 .It Cd options COMPAT_FREEBSD
 On those architectures that support it, this enables binary
 compatibility with
 .Fx
 applications built for the same architecture.
 At the moment this is limited to the i386 port.
 .It Cd options COMPAT_IBCS2
 On those architectures that support it, this enables binary
 compatibility with iBCS2 or SVR3 applications built for the same architecture.
 This is currently limited to the i386 and vax ports.
 .It Cd options COMPAT_IRIX
 On those architectures that support it, this enables binary
 compatibility with IRIX o32 binaries built for the same architecture.
 This feature is experimental, and it is currently limited to
 the sgimips port.
 .It Cd options COMPAT_MACH
 On those architectures that support it, this enables the emulation of
 Mach kernel traps for binaries built for the same architecture.
 This feature is highly experimental and it is currently
 limited to the i386 and powerpc ports of
 .Nx .
 .It Cd options COMPAT_OSF1
 On those architectures that support it, this enables binary
 compatibility with
 .Tn Digital
 .Ux
 .Po
 formerly
 .Tn OSF/1
 .Pc
 applications built for the same architecture.
 This is currently limited to the alpha port.
 .It Cd options COMPAT_NOMID
 Enable compatibility with
 .Xr a.out 5
 executables that lack a machine ID.
 This includes
 .Nx 0.8 Ns 's
 ZMAGIC format, and 386BSD and BSDI's
 QMAGIC, NMAGIC, and OMAGIC
 .Xr a.out 5
 formats.
 .It Cd options COMPAT_NETBSD32
 On those architectures that support it, this enables binary
 compatibility with 32-bit applications built for the same architecture.
 This is currently limited to the amd64 and sparc64 ports, and only
 applicable for 64-bit kernels.
 .It Cd options COMPAT_SVR4_32
 On those architectures that support it, this enables binary
 compatibility with 32-bit SVR4 applications built for the same architecture.
 This is currently limited to the sparc64 port, and only applicable for
 -bit kernels.
 .It Cd options COMPAT_AOUT_M68K
 On m68k architectures which have switched to ELF,
 this enables binary compatibility with
 .Nx Ns Tn /m68k
 .Xr a.out 5
 executables on
 .Nx Ns Tn /m68k
 ELF kernels.
 This handles alignment incompatibility of m68k ABI between
 a.out and ELF which causes the structure padding differences.
 Currently only some system calls which use
 .Va struct stat
 are adjusted and some binaries which use
 .Xr sysctl 3
 to retrieve network details would not work properly.
 .It Cd options EXEC_MACHO
 On those architectures that support it, this adds support for running
 Mach-O executables.
 This is currently limited to the i386 and powerpc ports of
 .Nx .
 .It Cd options EXEC_ELF_NOTELESS
 Run unidentified ELF binaries as
 .Nx
 binaries.
 This might be needed for very old
 .Nx
 ELF binaries on some archs.
 These old binaries didn't contain an appropriate
 .Li .note.netbsd.ident
 section, and thus can't be identified by the kernel as
 .Nx
 binaries otherwise.
 Beware - if this option is on, the kernel would run
 .Em any
 unknown ELF binaries as if they were
 .Nx
 binaries.
 .It Cd options P1003_1B_SEMAPHORE
 Includes kernel support for the standard C library
 .Pq libc
 functions that implement semaphores as specified in
 .St -p1003.1-96 .
 .El
 .Ss Debugging Options
 .Bl -ohang
 .It Cd options DDB
 Compiles in a kernel debugger for diagnosing kernel problems.
 See
 .Xr ddb 4
 for details.
 .Em NOTE :
 not available on all architectures.
 .It Cd options DDB_FROMCONSOLE=integer
 If set to non-zero, DDB may be entered by sending a break on a serial
 console or by a special key sequence on a graphics console.
 A value of "0" ignores console breaks or key sequences.
 If not explicitly specified, the default value is "1".
 Note that this sets the value of the
 .Em ddb.fromconsole
 .Xr sysctl 3
 variable which may be changed at run time -- see
 .Xr sysctl 8
 for details.
 .It Cd options DDB_HISTORY_SIZE=integer
 If this is non-zero, enable history editing in the kernel debugger
 and set the size of the history to this value.
 .It Cd options DDB_ONPANIC
 The default if not specified is
 .Dq 1
 - just enter into DDB.
 If set to
 .Dq 2
 the kernel will
 attempt to print out a stack trace before entering into DDB.
 If set to
 .Dq 0
 the kernel will attempt to print out a stack trace
 and reboot the system.
 If set to
 .Dq -1
 then neither a stack trace is printed or DDB entered -
 it is as if DDB were not compiled into the kernel.
 Note that this sets the value of the
 .Em ddb.onpanic
 .Xr sysctl 3
 variable which may be changed at run time -- see
 .Xr sysctl 8
 for details.
 .It Cd options DDB_COMMANDONENTER=string
 This option specify commands which will be executed on each entry to DDB.
 This sets the default value of the
 .Em ddb.commandonenter
 .Xr sysctl 3
 variable which may be changed at run time.
 .It Cd options DDB_BREAK_CHAR=integer
 This option overrides using break to enter the kernel debugger
 on the serial console.
 The value given is the ASCII value to be used instead.
 This is currently only supported by the com driver.
 .It Cd options DDB_VERBOSE_HELP
 This option adds more verbose descriptions to the
 .Em help
 command.
 .It Cd options KGDB
 Compiles in a remote kernel debugger stub for diagnosing kernel problems
 using the
 .Dq remote target
 feature of gdb.
 See
 .Xr gdb 1
 for details.
 .Em NOTE :
 not available on all architectures.
 .It Cd options KGDB_DEV
 Device number
 .Po
 as a
 .Dv dev_t
 .Pc
 of kgdb device.
 .It Cd options KGDB_DEVADDR
 Memory address of kgdb device.
 .It Cd options KGDB_DEVMODE
 Permissions of kgdb device.
 .It Cd options KGDB_DEVNAME
 Device name of kgdb device.
 .It Cd options KGDB_DEVRATE
 Baud rate of kgdb device.
 .It Cd makeoptions DEBUG="-g"
 The
 .Fl g
 flag causes
 .Pa netbsd.gdb
 to be built in addition to
 .Pa netbsd .
 .Pa netbsd.gdb
 is useful for debugging kernel crash dumps with gdb.
 See
 .Xr gdb 1
 for details.
 This also turns on
 .Em options DEBUG
 (which see).
 .It Cd options DEBUG
 Turns on miscellaneous kernel debugging.
 Since options are turned into preprocessor defines (see above),
 .Em options DEBUG
 is equivalent to doing a
 .Em #define DEBUG
 throughout the kernel.
 Much of the kernel has
 .Em #ifdef DEBUG
 conditionalized debugging code.
 Note that many parts of the kernel (typically device drivers) include their own
 .Em #ifdef XXX_DEBUG
 conditionals instead.
 This option also turns on certain other options,
 which may decrease system performance.
 .It Cd options DIAGNOSTIC
 Adds code to the kernel that does internal consistency checks.
 This code will cause the kernel to panic if corruption of internal data
 structures is detected.
 These checks can decrease performance up to 15%.
 .It Cd options KSTACK_CHECK_MAGIC
 Check kernel stack usage and panic if stack overflow is detected.
 This check is performance sensitive because it scans stack on each context
 switch.
 .It Cd options KTRACE
 Add hooks for the system call tracing facility, which allows users to
 watch the system call invocation behavior of processes.
 See
 .Xr ktrace 1
 for details.
 .It Cd options MSGBUFSIZE=integer
 This option sets the size of the kernel message buffer.
 This buffer holds the kernel output of
 .Fn printf
 when not (yet) read by
 .Xr syslogd 8 .
 This is particularly useful when the system has crashed and you wish to lookup
 the kernel output from just before the crash.
 Also, since the autoconfig output becomes more and more verbose,
 it sometimes happens that the message buffer overflows before
 .Xr syslogd 8
 was able to read it.
 Note that not all systems are capable of obtaining a variable sized message
 buffer.
 There are also some systems on which memory contents are not preserved
 across reboots.
 .It Cd options MALLOCLOG
 Enables an event log for
 .Xr malloc 9 .
 Useful for tracking down
 .Dq Data modified on freelist
 and
 .Dq multiple free
 problems.
 .It Cd options MALLOCLOGSIZE=integer
 Defines the number of entries in the malloc log.
 Default is 100000 entries.
 .It Cd options UVMHIST
 Enables the UVM history logs, which create in-memory traces of
 various UVM activities.
-These logs can be displayed be calling
+These logs can be displayed by calling
 .Fn uvmhist_dump
 or
 .Fn uvm_hist
 with appropriate arguments from DDB.
 See the kernel source file sys/uvm/uvm_stat.c for details.
 .It Cd options UVMHIST_PRINT
 Prints the UVM history logs on the system console as entries are added.
 Note that the output is
 .Em extremely
 voluminous, so this option is really only useful for debugging
 the very earliest parts of kernel initialization.
 .El
 .Ss File Systems
 .Bl -ohang
 .It Cd file-system FFS
 Includes code implementing the Berkeley Fast File System
 .Em ( FFS ) .
 Most machines need this if they are not running diskless.
 .It Cd file-system EXT2FS
 Includes code implementing the Second Extended File System
 .Em ( EXT2FS ) ,
 revision 0 and revision 1 with the
 .Em filetype
 and
 .Em sparse_super
 options.
 This is the most commonly used file system on the Linux operating system,
 and is provided here for compatibility.
 Some of the specific features of
 .Em EXT2FS
 like the "behavior on errors" are not implemented.
 This file system can't be used with UID or GID greater than 65535.
 See
 .Xr mount_ext2fs 8
 for details.
 .It Cd file-system LFS
 .Bq Em EXPERIMENTAL
 Include the Log-structured File System
 .Em ( LFS ) .
 See
 .Xr mount_lfs 8
 and
 .Xr newfs_lfs 8
 for details.
 .It Cd file-system MFS
 Include the Memory File System
 .Em ( MFS ) .
 This file system stores files in swappable memory, and produces
 notable performance improvements when it is used as the file store
 for
 .Pa /tmp
 and similar file systems.
 See
 .Xr mount_mfs 8
 for details.
 .It Cd file-system NFS
 Include the client side of the Network File System
 .Pq Tn NFS
 remote file sharing protocol.
 Although the bulk of the code implementing
 .Tn NFS
 is kernel based, several user level daemons are needed for it to work.
 See
 .Xr mount_nfs 8
 for details.
 .It Cd file-system CD9660
 Includes code for the
 .Tn ISO
 + Rock Ridge file system, which is the standard file system on many
 .Tn CD-ROM
 discs.
 Useful primarily if you have a
 .Tn CD-ROM
 drive.
 See
 .Xr mount_cd9660 8
 for details.
 .It Cd file-system MSDOSFS
 Includes the
 .Tn MS-DOS
 FAT file system, which is reportedly still used
 by unfortunate people who have not heard about
 .Nx .
 Also implements the
 .Tn Windows 95
 extensions to the same, which permit the use of longer, mixed case
 file names.
 See
 .Xr mount_msdos 8
 and
 .Xr fsck_msdos 8
 for details.
 .It Cd file-system NTFS
 .Bq Em EXPERIMENTAL
 Includes code for the
 .Tn Microsoft Windows NT
 file system.
 See
 .Xr mount_ntfs 8
 for details.
 .It Cd file-system FDESC
 Includes code for a file system, conventionally mounted on
 .Pa /dev/fd ,
 which permits access to the per-process file descriptor space via
 special files in the file system.
 See
 .Xr mount_fdesc 8
 for details.
 Note that this facility is redundant, and thus unneeded on most
 .Nx
 systems, since the
 .Xr fd 4
 pseudo-device driver already provides identical functionality.
 On most
 .Nx
 systems, instances of
 .Xr fd 4
 are mknoded under
 .Pa /dev/fd/
 and on
 .Pa /dev/stdin ,
 .Pa /dev/stdout ,
 and
 .Pa /dev/stderr .
 .It Cd file-system KERNFS
 Includes code which permits the mounting of a special file system
 (normally mounted on
 .Pa /kern )
 in which files representing various kernel variables and parameters
 may be found.
 See
 .Xr mount_kernfs 8
 for details.
 .It Cd file-system NULLFS
 Includes code for a loopback file system.
 This permits portions of the file hierarchy to be re-mounted in other places.
 The code really exists to provide an example of a stackable file system layer.
 See
 .Xr mount_null 8
 for details.
 .It Cd file-system OVERLAY
 Includes code for a file system filter.
 This permits the overlay file system to intercept all access to an underlying
 file system.
 This file system is intended to serve as an example of a stacking file
 system which has a need to interpose itself between an underlying file
 system and all other access.
 See
 .Xr mount_overlay 8
 for details.
 .It Cd file-system PORTAL
 .Bq Em EXPERIMENTAL
 Includes the portal file system.
 This permits interesting tricks like opening
 .Tn TCP
 sockets by opening files in the file system.
 The portal file system is conventionally mounted on
 .Pa /p
 and is partially implemented by a special daemon.
 See
 .Xr mount_portal 8
 for details.
 .It Cd file-system PROCFS
 Includes code for a special file system (conventionally mounted on
 .Pa /proc )
 in which the process space becomes visible in the file system.
 Among
 other things, the memory spaces of processes running on the system are
 visible as files, and signals may be sent to processes by writing to
 .Pa ctl
 files in the procfs namespace.
 See
 .Xr mount_procfs 8
 for details.
 .It Cd file-system UDF
 .Bq Em EXPERIMENTAL
 Includes code for the UDF file system commonly found on CD and DVD
 media but also more and more on USB sticks.
 Useful primarily if you have a CD or a DVD drive, be it a read-only
 or a rewritable device.
 Currently only supports read-access.
 See
 .Xr mount_udf 8
 for details.
 .It Cd file-system UMAPFS
 Includes a loopback file system in which user and group IDs may be
 remapped -- this can be useful when mounting alien file systems with
 different UIDs and GIDs than the local system.
 See
 .Xr mount_umap 8
 for details.
 .It Cd file-system UNION
 .Bq Em EXPERIMENTAL
 Includes code for the union file system, which permits directories to
 be mounted on top of each other in such a way that both file systems
 remain visible -- this permits tricks like allowing writing (and the
 deleting of files) on a read-only file system like a
 .Tn CD-ROM
 by mounting a local writable file system on top of the read-only file system.
 See
 .Xr mount_union 8
 for details.
 .It Cd file-system CODA
 .Bq Em EXPERIMENTAL
 Includes code for the Coda file system.
 Coda is a distributed file system like NFS and AFS.
 It is freely available, like NFS, but it functions much like AFS in being a
 .Dq stateful
 file system.
 Both Coda and AFS cache files on your local machine to improve performance.
 Then Coda goes a step further than AFS by letting you access the cached
 files when there is no available network, viz. disconnected laptops and
 network outages.
 In Coda, both the client and server are outside the kernel which makes
 them easier to experiment with.
 Coda is available for several UNIX and non-UNIX platforms.
 See http://www.coda.cs.cmu.edu for more details.
 .Em NOTE :
 You also need to enable the pseudo-device, vcoda, for the Coda
 file system to work.
 .It Cd file-system SMBFS
 .Bq Em EXPERIMENTAL
 Includes code for the SMB/CIFS file system.
 See
 .Xr mount_smbfs 8
 for details.
 .Em NOTE :
 You also need to enable the pseudo-device, nsmb, for the SMB
 file system to work.
 .It Cd file-system PTYFS
 .Bq Em EXPERIMENTAL
 Includes code for a special file system (normally mounted on
 .Pa /dev/pts )
 in which pseudo-terminal slave devices become visible in the file system.
 See
 .Xr mount_ptyfs 8
 for details.
 .It Cd file-system TMPFS
 Includes code for the efficient memory file system, normally used over
 .Pa /tmp .
 See
 .Xr mount_tmpfs 8
 for details.
 .It Cd file-system PUFFS
 .Bq Em EXPERIMENTAL
 Includes kernel support for the pass-to-userspace framework file system.
 It can be used to implement file system functionality in userspace.
 See
 .Xr puffs 3
 and
 .Xr puffs 4
 for more details.
 This option is required for sshfs:
 .Xr mount_psshfs 8 .
 .El
 .Ss File System Options
 .Bl -ohang
 .It Cd options MAGICLINKS
 Enables the expansion of special strings
 .Po
 beginning with
 .Dq @
 .Pc
 when traversing symbolic links.
 See
 .Xr symlink 7
 for a list of supported strings.
 Note that this option only controls the enabling of this feature
 by the kernel at boot-up.
 This feature can still be manipulated with the
 .Xr sysctl 8
 command regardless of the setting of this option.
 .It Cd options NFSSERVER
 Include the server side of the
 .Em NFS
 (Network File System) remote file sharing protocol.
 Although the bulk of the code implementing
 .Em NFS
 is kernel based, several user level daemons are needed for it to
 work.
 See
 .Xr mountd 8
 and
 .Xr nfsd 8
 for details.
 .It Cd options QUOTA
 Enables kernel support for file system quotas.
 See
 .Xr quotaon 8 ,
 .Xr edquota 8 ,
 and
 .Xr quota 1
 for details.
 Note that quotas only work on
 .Dq ffs
 file systems, although
 .Xr rpc.rquotad 8
 permits them to be accessed over
 .Em NFS .
 .It Cd options FFS_EI
 Enable ``Endian-Independent'' FFS support.
 This allows a system to mount an FFS file system created for another
 architecture, at a small performance cost for all FFS file systems.
 See also
 .Xr newfs 8 ,
 .Xr fsck_ffs 8 ,
 .Xr dumpfs 8
 for file system byte order status and manipulation.
 .It Cd options FFS_NO_SNAPSHOT
 Disable the
 .Dq file system snapshot
 support in FFS file systems.
 Maybe useful for install media kernels, small memory systems and
 embedded systems which don't require the snapshot support.
 .It Cd options WAPBL
 Enable
 .Dq Write Ahead Physical Block Logging file system journaling .
 This provides rapid file system consistency checking after a system outage.
 It also provides better general use performance over regular FFS.
 See also
 .Xr wapbl 4 .
 .It Cd options NVNODE=integer
 This option sets the size of the cache used by the name-to-inode translation
 routines, (a.k.a. the
 .Fn namei
 cache, though called by many other names in the kernel source).
 By default, this cache has
 .Dv NPROC
 (set as 20 + 16 * MAXUSERS) * (80 + NPROC / 8) entries.
 A reasonable way to derive a value of
 .Dv NVNODE ,
 should you notice a large number of namei cache misses with a tool such as
 .Xr systat 1 ,
 is to examine your system's current computed value with
 .Xr sysctl 8 ,
 (which calls this parameter "kern.maxvnodes") and to increase this value
 until either the namei cache hit rate improves or it is determined that
 your system does not benefit substantially from an increase in the size of
 the namei cache.
 .It Cd options NAMECACHE_ENTER_REVERSE
 Causes the namei cache to always enter a reverse mapping (vnode -\*[Gt] name)
 as well as a normal one.
 Normally, this is already done for directory vnodes, to speed up the getcwd
 operation.
 This option will cause longer hash chains in the reverse cache, and thus
 slow down getcwd somewhat.
 However, it does make vnode -\*[Gt] path translations possible in some cases.
 For now, only useful if strict /proc/#/maps emulation for Linux binaries is
 required.
 .It Cd options EXT2FS_SYSTEM_FLAGS
 This option changes the behavior of the APPEND and IMMUTABLE flags
 for a file on an
 .Em EXT2FS
 file system.
 Without this option, the superuser or owner of the file can
 set and clear them.
 With this option, only the superuser can set them, and
 they can't be cleared if the securelevel is greater than 0.
 See also
 .Xr chflags 1
 and
 .Xr secmodel_securelevel 9 .
 .It Cd options NFS_BOOT_BOOTP
 Enable use of the BOOTP protocol (RFCs 951 and 1048) to get configuration
 information if NFS is used to mount the root file system.
 See
 .Xr diskless 8
 for details.
 .It Cd options NFS_BOOT_DHCP
 Same as
 .Dq NFS_BOOT_BOOTP ,
 but use the DHCP extensions to the
 BOOTP protocol (RFC 1541).
 .It Cd options NFS_BOOT_BOOTP_REQFILE
 Specifies the string sent in the bp_file field of the BOOTP / DHCP
 request packet.
 .It Cd options NFS_BOOT_BOOTPARAM
 Enable use of the BOOTPARAM protocol, consisting of RARP and
 BOOTPARAM RPC, to get configuration information if NFS
 is used to mount the root file system.
 See
 .Xr diskless 8
 for details.
 .It Cd options NFS_BOOT_RWSIZE=value
 Set the initial NFS read and write sizes for diskless-boot requests.
 The normal default is 8Kbytes.
 This option provides a way to lower the value (e.g., to 1024 bytes)
 as a workaround for buggy network interface cards or boot PROMs.
 Once booted, the read and write request sizes can be increased by
 remounting the file system.
 See
 .Xr mount_nfs 8
 for details.
 .It Cd options NFS_V2_ONLY
 Reduce the size of the NFS client code by omitting code that's only required
 for NFSv3 and NQNFS support, leaving only that code required to use NFSv2
 servers.
 .It Cd options SOFTDEP
 Enable kernel support for soft-dependencies in FFS file systems.
 Softdep improves write performance by aggregating and properly
 ordering disk metadata writes, achieving near-asynchronous write
 performance while maintaining the file system consistency of
 synchronous writes.
 Soft-dependencies are enabled on a per-mount basis.
 See
 .Xr mount 8
 for details.
 .It Cd options UFS_DIRHASH
 Increase lookup performance by maintaining in-core hash tables
 for large directories.
 .El
 .Ss Buffer queue strategy options
 The following options enable alternative buffer queue strategies.
 .Bl -ohang
 .It Cd options BUFQ_READPRIO
 Enable experimental buffer queue strategy for disk I/O.
 In the default strategy, outstanding disk requests are ordered by
 sector number and sent to the disk, regardless of whether the
 operation is a read or write; this option gives priority to issuing
 read requests over write requests.
 Although requests may therefore be issued out of sector-order, causing
 more seeks and thus lower overall throughput, interactive system
 responsiveness under heavy disk I/O load may be improved, as processes
 blocking on disk reads are serviced sooner (file writes typically
 don't cause applications to block).
 The performance effect varies greatly depending on the hardware, drive
 firmware, file system configuration, workload, and desired performance
 trade-off.
 Systems using drive write-cache (most modern IDE disks, by default)
 are unlikely to benefit and may well suffer; such disks acknowledge
 writes very quickly, and optimize them internally according to
 physical layout.
 Giving these disks as many requests to work with as possible (the
 standard strategy) will typically produce the best results, especially
 if the drive has a large cache; the drive will silently complete
 writes from cache as it seeks for reads.
 Disks that support a large number of concurrent tagged requests (SCSI
 disks and many hardware RAID controllers) expose this internal
 scheduling with tagged responses, and don't block for reads; such
 disks may not see a noticeable difference with either strategy.
 However, if IDE disks are run with write-cache disabled for safety,
 writes are not acknowledged until actually completed, and only one
 request can be outstanding; a large number of small writes in one
 locality can keep the disk busy, starving reads elsewhere on the disk.
 Such systems are likely to see the most benefit from this option.
 Finally, the performance interaction of this option with ffs soft
 dependencies can be subtle, as that mechanism can drastically alter
 the workload for file system metadata writes.
 .It Cd options BUFQ_PRIOCSCAN
 Enable another buffer queue strategy for disk I/O, per-priority cyclical scan.
 .It Cd options NEW_BUFQ_STRATEGY
 Synonym of
 .Em BUFQ_READPRIO .
 .El
 .Ss Miscellaneous Options
 .Bl -ohang
 .It Cd options LKM
 Enable loadable kernel modules.
 See
 .Xr lkm 4
 for details.
 .Em NOTE :
 not available on all architectures.
 .It Cd options MEMORY_DISK_DYNAMIC
 This option makes the
 .Xr md 4
 .Tn RAM
 disk size dynamically sized.
 It is incompatible with
 .Xr mdsetimage 8 .
 .It Cd options MEMORY_DISK_HOOKS
 This option allows for some machine dependent functions to be called when
 the
 .Xr md 4
 .Tn RAM
 disk driver is configured.
 This can result in automatically loading a
 .Tn RAM
 disk from floppy on open (among other things).
 .It Cd options MEMORY_DISK_IS_ROOT
 Forces the
 .Xr md 4
 .Tn RAM
 disk to be the root device.
 This can only be overridden when
 the kernel is booted in the 'ask-for-root' mode.
 .It Cd options MEMORY_DISK_ROOT_SIZE=integer
 Allocates the given number of 512 byte blocks as memory for the
 .Xr md 4
 .Tn RAM
 disk, to be populated with
 .Xr mdsetimage 8 .
 .It Cd options MEMORY_DISK_SERVER=0
 Do not include the interface to a userland memory disk server process.
 Per default, this option is set to 1, including the support code.
 Useful for install media kernels.
 .It Cd options MODULAR
 Enables the new framework for kernel modules, which will eventually
 replace LKMs.
 This adds an in-kernel linker and loader, and requires userland support
 to be useful.
 See the
 .Sy MKMODULAR
 variable in
 .Xr mk.conf 5
 for more details.
 .It Cd options VND_COMPRESSION
 Enables the
 .Xr vnd 4
 driver to also handle compressed images.
 See
 .Xr vndcompress 1 ,
 .Xr vnd 4
 and
 .Xr vnconfig 8
 for more information.
 .It Cd options TFTPROOT
 Download the root memory disk through TFTP at root mount time.
 This enables the use of a root
 .Tn RAM
 disk without requiring it to be embedded in the kernel using
 .Xr mdsetimage 8 .
 The
 .Tn RAM
 disk name is obtained using DHCP's filename parameter.
 This option requires
 .Em MEMORY_DISK_HOOKS ,
 .Em MEMORY_DISK_DYNAMIC ,
 and
 .Em MEMORY_DISK_IS_ROOT .
 It is incompatible with
 .Em MEMORY_DISK_ROOT_SIZE .
 .It Cd options MALLOC_NOINLINE
 Time critical fixed size memory allocation is performed with
 .Fn MALLOC
 and
 .Fn FREE .
 Normally these expand to inline code, but with
 .Em MALLOC_NOINLINE
 these call the normal
 .Fn malloc
 and
 .Fn free
 functions.
 Useful for install media kernels, small memory systems and embedded systems.
 .It Cd options HZ=integer
 On ports that support it, set the system clock frequency (see
 .Xr hz 9 )
 to the supplied value.
 Handle with care.
 .It Cd options NTP
 Turns on in-kernel precision timekeeping support used by software
 implementing
 .Em NTP
 (Network Time Protocol, RFC 1305).
 The
 .Em NTP
 option adds an in-kernel Phase-Locked Loop (PLL) for normal
 .Em NTP
 operation, and a Frequency-Locked Loop (FLL) for intermittently-connected
 operation.
 .Xr ntpd 8
 will employ a user-level PLL when kernel support is unavailable,
 but the in-kernel version has lower latency and more precision, and
 so typically keeps much better time.
 The interface to the kernel
 .Em NTP
 support is provided by the
 .Xr ntp_adjtime 2
 and
 .Xr ntp_gettime 2
 system calls, which are intended for use by
 .Xr ntpd 8
 and are enabled by the option.
 On systems with sub-microsecond resolution timers, or where (HZ / 100000)
 is not an integer, the
 .Em NTP
 option also enables extended-precision arithmetic to keep track of
 fractional clock ticks at NTP time-format precision.
 .It Cd options PPS_SYNC
 This option enables a kernel serial line discipline for receiving time
 phase signals from an external reference clock such as a radio clock.
 (The
 .Em NTP
 option (which see) must be on if the
 .Em PPS_SYNC
 option is used.)
 Some reference clocks generate a Pulse Per Second (PPS) signal in
 phase with their time source.
 The
 .Em PPS
 line discipline receives this signal on either the data leads
 or the DCD control lead of a serial port.
 .Em NTP
 uses the PPS signal to discipline the local clock oscillator to a high
 degree of precision (typically less than 50 microseconds in time and
 .1 ppm in accuracy).
 .Em PPS
 can also generate a serial output pulse when the system receives a PPS
 interrupt.
 This can be used to measure the system interrupt latency and thus calibrate
 .Em NTP
 to account for it.
 Using
 .Em PPS
 usually requires a
 gadget box
 to convert from TTL to RS-232 signal levels.
 The gadget box and PPS are described in more detail in the HTML documentation
 for
 .Xr ntpd 8
 in
 .Pa /usr/share/doc/html/ntp .
 .It Cd options SETUIDSCRIPTS
 Allows scripts with the setuid bit set to execute as the effective
 user rather than the real user, just like binary executables.
 .Pp
 .Em NOTE :
 Using this option will also enable
 .Em options FDSCRIPTS
 .It Cd options FDSCRIPTS
 Allows execution of scripts with the execute bit set, but not the
 read bit, by opening the file and passing the file descriptor to
 the shell, rather than the filename.
 .Pp
 .Em NOTE :
 Execute only (non-readable) scripts will have
 .Va argv[0]
 set to
 .Pa /dev/fd/* .
 What this option allows as far as security is
 concerned, is the ability to safely ensure that the correct script
 is run by the interpreter, as it is passed as an already open file.
 .It Cd options PUCCN
 Enables treating serial ports found on PCI boards
 .Xr puc 4
 as potential console devices.
 The method for choosing such a console device is port dependent.
 .It Cd options RTC_OFFSET=integer
 The kernel (and typically the hardware battery backed-up clock on
 those machines that have one) keeps time in
 .Em UTC
 (Universal Coordinated Time, once known as
 .Em GMT ,
 or Greenwich Mean Time)
 and not in the time of the local time zone.
 The
 .Em RTC_OFFSET
 option is used on some ports (such as the i386) to tell the kernel
 that the hardware clock is offset from
 .Em UTC
 by the specified number of minutes.
 This is typically used when a machine boots several operating
 systems and one of them wants the hardware clock to run in the
 local time zone and not in
 .Em UTC ,
 e.g.
 .Em RTC_OFFSET=300
 means
 the hardware clock is set to US Eastern Time (300 minutes behind
 .Em UTC ) ,
 and not
 .Em UTC .
 (Note:
 .Em RTC_OFFSET
 is used to initialize a kernel variable named
 .Va rtc_offset
 which is the source actually used to determine the clock offset, and
 which may be accessed via the kern.rtc_offset sysctl variable.
 See
 .Xr sysctl 8
 and
 .Xr sysctl 3
 for details.
 Since the kernel clock is initialized from the hardware clock very
 early in the boot process, it is not possible to meaningfully change
 .Va rtc_offset
 in system initialization scripts.
 Changing this value currently may only be done at kernel compile
 time or by patching the kernel and rebooting).
 .Pp
 .Em NOTE :
 Unfortunately, in many cases where the hardware clock
 is kept in local time, it is adjusted for Daylight Savings
 Time; this means that attempting to use
 .Em RTC_OFFSET
 to let
 .Nx
 coexist with such an operating system, like Windows,
 would necessitate changing
 .Em RTC_OFFSET
 twice a year.
 As such, this solution is imperfect.
 .It Cd options KMEMSTATS
 The kernel memory allocator,
 .Xr malloc 9 ,
 will keep statistics on its performance if this option is enabled.
 Unfortunately, this option therefore essentially disables the
 .Fn MALLOC
 and
 .Fn FREE
 forms of the memory allocator, which are used to enhance the performance
 of certain critical sections of code in the kernel.
 This option therefore can lead to a significant decrease in the
 performance of certain code in the kernel if enabled.
 Examples of such code include the
 .Fn namei
 routine, the
 .Xr ccd 4
 driver,
 and much of the networking code.
 .It Cd options MAXUPRC=integer
 Sets the soft
 .Dv RLIMIT_NPROC
 resource limit, which specifies the maximum number of simultaneous
 processes a user is permitted to run, for process 0;
 this value is inherited by its child processes.
 It defaults to
 .Em CHILD_MAX ,
 which is currently defined to be 160.
 Setting
 .Em MAXUPRC
 to a value less than
 .Em CHILD_MAX
 is not permitted, as this would result in a violation of the semantics of
 .St -p1003.1-90 .
 .It Cd options NOFILE=integer
 Sets the soft
 .Dv RLIMIT_NOFILE
 resource limit, which specifies the maximum number of open
 file descriptors for each process;
 this value is inherited by its child processes.
 It defaults to
 .Em OPEN_MAX ,
 which is currently defined to be 64.
 .It Cd options MAXFILES=integer
 Sets the default value of the
 .Em kern.maxfiles
 sysctl variable, which indicates the maximum number of files that may
 be open in the system.
 .It Cd options DEFCORENAME=string
 Sets the default value of the
 .Em kern.defcorename
 sysctl variable, otherwise it is set to
 .Nm %n.core .
 See
 .Xr sysctl 8
 and
 .Xr sysctl 3
 for details.
 .It Cd options RASOPS_CLIPPING
 Enables clipping within the
 .Nm rasops
 raster-console output system.
 .Em NOTE :
 only available on architectures that use
 .Nm rasops
 for console output.
 .It Cd options RASOPS_SMALL
 Removes optimized character writing code from the
 .Nm rasops
 raster-console output system.
 .Em NOTE :
 only available on architectures that use
 .Nm rasops
 for console output.
 .It Cd options INCLUDE_CONFIG_FILE
 Embeds the kernel config file used to define the kernel in the kernel
 binary itself.
 The embedded data also includes any files directly included by the config
 file itself, e.g.
 .Pa GENERIC.local
 or
 .Pa std.$MACHINE .
 The embedded config file can be extracted from the resulting kernel with
 .Xr config 1
 .Fl x ,
 or by the following command:
 .Bd -literal -offset indent
 strings netbsd | sed -n 's/^_CFG_//p' | unvis
 .Ed
 .It Cd options INCLUDE_JUST_CONFIG
 Similar to the above option, but includes just the actual config file,
 not any included files.
 .It Cd options PIPE_SOCKETPAIR
 Use slower, but smaller socketpair(2)-based pipe implementation instead
 of default faster, but bigger one.
 Primarily useful for installation kernels.
 .It Cd options USERCONF
 Compiles in the in-kernel device configuration manager.
 See
 .Xr userconf 4
 for details.
 .It Cd options PERFCTRS
 Compiles in kernel support for CPU performance-monitoring counters.
 See
 .Xr pmc 1
 for details.
 .Em NOTE :
 not available on all architectures.
 .It Cd options SYSCALL_STATS
 Count the number of times each system call number is called.
 The values can be read through the sysctl interface and displayed using
 .Xr systat 1 .
 .Em NOTE :
 not yet available on all architectures.
 .It Cd options SYSCALL_TIMES
 Count the time spent (using
 .Fn cpu_counter32 )
 in each system call.
 .Em NOTE :
 Using this option will also enable
 .Cd options SYSCALL_STATS .
 .It Cd options SYSCALL_TIMES_HASCOUNTER
 Force use of
 .Fn cpu_counter32
 even if
 .Fn cpu_hascounter
 reports false.
 Useful for systems where the cycle counter doesn't run at a constant rate
 (e.g. Soekris boxes).
 .It Cd options XSERVER
 Compiles in kernel support for
 .Tn X11
 on architectures that still use (or can use) the legacy
 .Em pccons
 console drivers rather than
 .Xr wscons 4 .
 These include bebox, i386, shark.
 .It Cd options XSERVER_DDB
 A supplement to XSERVER that adds support for entering
 .Xr ddb 4
 while in
 .Tn X11 .
 .It Cd options FILEASSOC
 Support for
 .Xr fileassoc 9 .
 .It Cd options FILEASSOC_NHOOKS=integer
 Number of storage slots per file for
 .Xr fileassoc 9 .
 Default is 4.
 .It Cd options I2C_SCAN
 Scan each i2c bus to determine which addresses respond.
 .Pp
 .Em WARNING !
 Using this option can access some devices in such a
 manner as to leave them in an unstable or unuseable state, and
 can prevent those devices from being properly matched and/or
 attached.
 It can also lock up the entire i2c bus and even
 prevent a machine from completing the boot process.
 Don't use this option unless you know what you're doing and can
 accept all sorts of unforseen consequences.
 .El
 .Ss Networking Options
 .Bl -ohang
 .It Cd options GATEWAY
 Enables
 .Em IPFORWARDING
 (which see)
 and (on most ports) increases the size of
 .Em NMBCLUSTERS
 (which see).
 In general,
 .Em GATEWAY
 is used to indicate that a system should act as a router, and
 .Em IPFORWARDING
 is not invoked directly.
 (Note that
 .Em GATEWAY
 has no impact on protocols other than
 .Tn IP ,
 such as
 .Tn CLNP
 or
 .Tn XNS ) .
 .Em GATEWAY
 option also compiles IPv4 and IPv6 fast forwarding code into the kernel.
 .It Cd options ICMPPRINTFS
 The
 .Em ICMPPRINTFS
 option will enable debugging information to be printed about
 the
 .Xr icmp 4
 protocol.
 .It Cd options IPFORWARDING=value
 If
 .Em value
 is 1 this enables IP routing behavior.
 If
 .Em value
 is 0 (the default), it disables it.
 The
 .Em GATEWAY
 option sets this to 1 automatically.
 With this option enabled, the machine will forward IP datagrams destined
 for other machines between its interfaces.
 Note that even without this option, the kernel will
 still forward some packets (such as source routed packets) -- removing
 .Em GATEWAY
 and
 .Em IPFORWARDING
 is insufficient to stop all routing through a bastion host on a
 firewall -- source routing is controlled independently.
 To turn off source routing, use
 .Em options IPFORWSRCRT=0
 (which see).
 Note that IP forwarding may be turned on and off independently of the
 setting of the
 .Em IPFORWARDING
 option through the use of the
 .Em net.inet.ip.forwarding
 sysctl variable.
 If
 .Em net.inet.ip.forwarding
 is 1, IP forwarding is on.
 See
 .Xr sysctl 8
 and
 .Xr sysctl 3
 for details.
 .It Cd options IPFORWSRCRT=value
 If
 .Em value
 is set to zero, source routing of IP datagrams is turned off.
 If
 .Em value
 is set to one (the default) or the option is absent, source routed IP
 datagrams are forwarded by the machine.
 Note that source routing of IP packets may be turned on and off
 independently of the setting of the
 .Em IPFORWSRCRT
 option through the use of the
 .Em net.inet.ip.forwsrcrt
 sysctl variable.
 If
 .Em net.inet.ip.forwsrcrt
 is 1, forwarding of source routed IP datagrams is on.
 See
 .Xr sysctl 8
 and
 .Xr sysctl 3
 for details.
 .It Cd options IFA_STATS
 Tells the kernel to maintain per-address statistics on bytes sent
 and received over (currently) Internet and AppleTalk addresses.
 .\"This can be a fairly expensive operation, so you probably want to
 .\"keep this disabled.
 The option is not recommended as it degrades system stability.
 .It Cd options IFQ_MAXLEN=value
 Increases the allowed size of the network interface packet queues.
 The default queue size is 50 packets, and you do not normally need
 to increase it.
 .It Cd options IPSELSRC
 Includes support for source-address selection policies.
 See
 .Xr in_getifa 9 .
 .It Cd options MROUTING
 Includes support for IP multicast routers.
 You certainly want
 .Em INET
 with this.
 Multicast routing is controlled by the
 .Xr mrouted 8
 daemon.
 See also option
 .Cd PIM .
 .It Cd options PIM
 Includes support for Protocol Independent Multicast (PIM) routing.
 You need
 .Em MROUTING
 and
 .Em INET
 with this.
 Software using this can be found e.g. in
 .Pa pkgsrc/net/xorp .
 .It Cd options INET
 Includes support for the
 .Tn TCP/IP
 protocol stack.
 You almost certainly want this.
 See
 .Xr inet 4
 for details.
 .It Cd options INET6
 Includes support for the
 .Tn IPv6
 protocol stack.
 See
 .Xr inet6 4
 for details.
 Unlike
 .Em INET ,
 .Em INET6
 enables multicast routing code as well.
 This option requires
 .Em INET
 at this moment, but it should not.
 .It Cd options ND6_DEBUG